The success of a marriage nearly always depends on the expectations of the parties going in. If you believe marriage is going to be a rose garden of happy trips to the beach interspersed with moonlight dinners and foot massages, you're more likely to end up cheating on your spouse when that doesn't materialize. If you believe marriage is a mechanism for changing your potential spouse, you're likely to end up estranged. If you believe that marriage is about a lifelong union devoted to self-improvement and the creation and rearing of children, you're likely to make decisions that lead to that outcome.

The same is true in politics.

Americans don't trust politicians. That's for good reason. Politicians fib to get elected; they pander to particular constituencies; they leave principle at the door in favor of convenience in order to maintain power and position. But they do not, at least not that often, murder people and collude with foreign governments.

But thanks to popular culture, that's exactly what many Americans think politicians do. If you watch "House of Cards," you're likely to believe that top-level politicians off each other on a regular basis -- and you might be more willing to believe conspiracy theories about the murder of former Democratic National Committee staffer Seth Rich. If you've seen "The Manchurian Candidate," you're more likely to believe that either former President Obama or President Trump is one.

President Trump, who was a layman until he became president, obviously believes a lot of the pop culture mythology surrounding politics. That's why he told Bill O'Reilly that it's not that big a deal that Russian President Vladimir Putin kills his political opponents. "You think our country's so innocent?" he said in jaw-dropping fashion. That's why Trump believes that politics is such a "dirty business" -- dirtier, even, than Manhattan real estate, where Trump worked with mafia figures. Politics, in Trump's mind, is the lowest of the low.

That means a more corrupt administration. If you believe, as Trump assuredly does, that anybody would take a meeting with Russian government figures to dig up dirt on an opponent, then you'll do it, too. Why be martyred just because you were too holy to get down in the mud? If you think that Hillary Clinton is the way politics is done -- we shouldn't be outraged because she's just another politician -- then why not play by the same rules?

Voters like to believe, as Trump does, that politics is filthy, because we refuse to acknowledge that in a representative republic, we're the ultimate sources of blame. We keep electing these moral idiots. We keep voting for them, demanding that they give us things and suggesting that they've "sold out" if they don't. We're the ones who decry crony capitalism while complaining that the local factory will leave unless the government "does something."

Politicians like to believe, as Trump does, that politics is sordid, because that's a tailor-made excuse for participating in bad behavior. It's also an excuse for legislating morality, as Sen. John McCain tried to do with campaign finance reform -- you can use the public distrust of politicians to restrict the political speech of citizens, all in the name of "cleaning up the system."

It's always easier to shift our vision of politics than to shift our vision of ourselves.

And so, we get the politics we deserve. Our belief that politics is a squalid affair finds realization in our politicians, who reflect that view; and in ourselves, as we vote for those politicians. And then we're surprised when politics seems to grow more and more disreputable.